Vacuum tube amplifier



June 28,1949.

R. C. MOORE VACUUM TUBE AMPLIFIER Filed" Jan. 8, 1945 Patented June 28, 1949 VAGUUMLTUBE AMPLIFIER.

RobertsC. Moore; Philadelphia, Pa., assignor, by

mesne. assignments; to Philco Corporation, Philadelphia, Ba-.,., a corporation: of- Pennsyl-.--

Vania Application January '8, 1945; SerialN'o; 5712935 (01. Nth-M19.

Claims. L

This. inventiom relates to an: el'ectr-ical apparatus and. particularly to an amplifier having large; voltage output. This =invention in particulan -relatesto an amplifier having a Wide irequency-response as in-vidoworkwhere substantial: voltage swings: are" desired.

Asrisxwell'c known, the extended response of a video amplifier precludes the use of transformers fonboosting: voltagesw-ings. In fact, video amplifi'ers asapractical necessity arerestricted to the -mpedance capacity coupled type. 'obta-inrlarge voltage variations in such an amplifier; it: is necessary to-have substantial current in. a: moder-ate size resistor;

lnrorderyto provide highvoltage output, it'has been: necessary to employ tubeshaving substantial power handling ability. Apart from the In order to:

expense; of: such it tubes, the large size of the tubes.

andv associated equipment results in high distributed capacitance and tends to reduce the high frequency response.

This:invention provides a meanswherebyvideo amplification and substantial voltage variations are-obtained with conventional equipment. While distributed input capacitance of apparatus utilizingtheinventioniscomparable with that off asimple video amplifier, the invention provides means Torr-obtaining increased amplification. The. invention has many" advantages in addition tothe elimination of high' power equipment. Thus,

the'output capacitance of a group of vacuum tubes is greatlyreduceddue tothe' disposition of! the tubes.

Theinvention-in general contemplates the disposition of a number of" tubes'in seriesv one above;

the other with regard to potential. The input signal'iis applied to-the bottomtube in the series. Each time transmits the amplified'signal to the.

tube above it; the connections being so arranged. that the gain-in each tube is added to that of the.

tube below it. More specifically, the anode of a lowertube'is conductively connected .to thecathode-offthe next higher tube, while the grid of the. higher tube is connected from apoint on the-load;

resistor of the lower. tube. arrangement,. proper phasereversal is-obtained; so'that'the gain is cumulative. This is tobeadis By virtueof: thise tir'igguishedv from 1 asimple. superposition: of: tubes wherein the gain of all the: tubes; is. no greater.

than. the-gain of; any one.

In order that a more detailed .descriptionxof the invention may be given, reference. will now be madeto. the drawings whereinEigure 1' is a=circult diagram.of. one-.,fornr ofsthe invention; Fig- 55 input and-aneifective amplification of ten in ure 2 is a circuit diagram of a modified form"uti--- lizinan common powersupply.

R'eierr-ing first to=- Figure '1, the lowermost vacuum tube" N1 has cathode ll grounded at Tube I'll has control grid l3- connected through grid resistance M-to bias source [-5 and thence toground. It will be noted that bias source- Hi has its negative terminal connected to the grid While the positive terminal is grounded; An input circuit to control grid- 8- may be provid'ed through blocking: condenser Hi the other 't'ermi n'al of'the input being at groundi Vacuum" tube I 0 has anode connected;

through-load resistance 2+ tothe positive termrnal of" plate voltage source 22- whose negative terminalmaybe grounded; It isunderstood-that".

plate voltage source 22 and bias source I? have" suitable voltage ranges for energizing vacuum tube III? From anode ZD-ofvacuum tube l0, lead iii-goes to cathode of vacuum tube 21; this being the next one'abovetube l0: Cathode 2'6 isconn'ectedi through bias source 28 and grid resistance 29 to control grid 30. An' input" circuit" to control grid 1 30''- is provided" through blocking condenser 3i by" lead 32""connectedto' point 33, on load resistor 21;.

V'acuum tube zl has-"anode 35" conne'cted'throug'h.

loadresistancedfito plate voltage source Bland thence backtocathode 26;

Anode-354s connected by'lead' 4'0 to cathode 41" of thenext' highertube'fl; Cathode M is*conheated through bias s ource "43 and grid resistance M te its-controlgrid451' An input circuit tO'COIlF- trol grid 45 is provided through blocking condenser 46 and lead 41 taken from point tB on load" resistance Vacuumtube 42- 'has' anode 50 connected-throughload resistance 51 and plate,

voltage source -52* to cathode: 41.

The output'of-the systemmay be taken between anode-Ett-andgroundl Tubes I 0, 27, and42 need" notnecessarily be of the same type: However,. since-all the tubes handl'ethe same order of space current, it is preferably, that the tubes be similar. Points 33' and l'fi on the load resistors are pref erably so chosen'as-"to impress the proper portionv of the voltage swing? due toea'ch' tube to the grid. of the tube above it. In. practice, the. voltagevariations between point33 and anode 20 should.- be approximately-equal to the voltage swingdue. to-signals at control grid?" l3. The same is true. of thevoltage swing-between point dfland'anode.

35;- The magnitude of load'resistors 2|, 35; and.

5! will all be determined by usual design methods.

For purposes of explanation, acne-volt signal,

3 each tube and circuit will be assumed. If grid l3 goes towards positive by one volt, anode 20 will go negative by ten volts. This means that cathode 26 will go negative by ten volts. On the other hand, point 33 will only go negative nine volts, so that grid 30 will go positive by one volt with respect to its cathode. Thus, cathode 26 will be minus ten volts and grid 30 will be minus nine volts. This will result in anode 35 being at minus twenty volts. Point 48 will be at minus nineteen volts. Thus, grid 45 and cathode 41 will be respectively at minus nineteen volts and minus twenty volts, so that anode 50 will be at minus thirty volts.

It is understood that, in this analysis, only alternating potentials are considered. It i clear that the gain of the entire system is the gain of all the tubes added together. If the tubes are all alike and the gain is the same, then the total gain will be the gain for one tube times the number of tubes.

The input capacitance, as seen from blocking condenser ll, will be the input capacitance of a single tube. The output capacitance of the system of Fig. 1 consists of the output capacitances of the tubes 1 I, 21 and 42, each of which is across it respective resistor 2|, 36 and The total output capacitance is thus reduced by the series relation.

The system described above suffers from the requirement of a plurality of power supplies. Due to the physical size of such power supplies, the distributed capacitance may be objectionably high, in addition to the inconvenience and expense of separate power supplies. In order to eliminate this, the system shown in Figure 2 is provided.

In this system, the input circuit goes to blocking condenser 60 thence to control grid 6| of the lowest tube 62. Tube 62 has cathode 63 connected to ground through bias resistance 64. Control grid BI is connected to ground through resistance 65. Tube 62 has anode 66 connected through condenser 61 to junction point 68. Junction point 68 has connected thereto resistances 69 and it in series. Resistance is connected to source 10' of 13+ potential. This B potential source should have a sufiiciently high value, so that the tubes in series may be properly energized. Thus, assuming that three tubes are used as shown here and that the tubes are similar and operated similarly, then the B supply should have a voltage at least three times that necessary to operate one tube, but the current requirements of the source are moderate.

From point ll, which is the junction between resistors 69 and 10, lead 12 goes to blocking condenser 13 and the latter is connected to control grid iii of succeeding tube 16. Tube 16 has cathode ll connected through bias resistance 18 to anode 66. Anode 66 is connected through grid resistance 19 to control grid (5. Vacuum tube 16 has anode 80 connected through blocking condenser 8! to point 92. Point 82 is connected to junction point 68 through resistances 83 and 84 in series. From junction 85 between resistors 83 and 84, lead 8% goes through blocking condenser 81 to control grid 88 of tube 89. Cathode 90 is connected through bias resistance 9| to anode 80, and is connected through grid resistance 92 to control grid 88. Tube 89 has anode 93 connected by lead 94 to resistance 95 and then to junction point 82.

The output of the entire system is taken between ground and anode lead .94 while the input is between ground and blocking condenser 69. The entire series of resistors between anode lead 94 and potential source Ill constitutes a load resistor for the amplifier system. By-pass condenser 96 may be connected between ground (or cathode E3) and resistance 10.

The junction points between load resistors for one tube are so chosen that the signals impressed on the grid of the next tube equal in amplitude the signals on the tube below. Thus resistances 69 and 10 are so chosen that point II feeds signals to control grid 15 equal in amplitude, with respect to cathode 11, to the signals impressed upon grid 6|. It is thus possible to omit blocking condensers 61 and 8! completely and their connections to the anodes of tubes 62 and 16, when the tubes are all similar and have substantially equal input signals.

Any type vacuum tube may be used that is desired. Thus, it is possible to use tetrodes or pentodes and obtain the benefits of low output capacitance and high voltage output. In the event that pentodes are used, the usual connections between electrodes and the load resistance will be made. Thus, tube 89 has resistance as the load, and any suitable potential sources bypassed to the cathodes may be taken for connection to the additional electrodes of a pentode.

It is understood that the heaters in the various tubes will be suitably energized. The cathodes of the various tubes are all at different potentials.

This would mean that the heaters would either have to be at one potential, if a single current source were used, or might have individual filament windings isolated from the remaining filament windings. possible to maintain a substantial difference in potential between the heater and cathode so that a common heating source may be used. However, separate heating sources suitably isolated from each other may be preferable, particularly if provision is made for keeping the capacity to ground of the winding or other source at a minimum.

The low frequency response of the system may be :made as good as any system having comparable amplification. In fact, the limiting factor for low frequency response is the size of the coupling condenser. In the system shown here, the number of coupling condensers is equal to the number of tubes used. Due to modern improvement in condensers, it is possible to obtain substantial capacitance with small physical size so that, in practice, the high frequency response for the system described here may be carried to about the same place in the frequency spectrum as in a conventional system.

By virtue of the invention herein, an amplifying system is provided having substantial voltage output and desirable characteristics. A video amplifier inherently has a load whose impedance is small in comparison to the impedance of the amplifier tube. In the system herein disclosed, the load resistor for the entire system may be considered as the normal individual tube loads connected in series, each tube driving the tube above it in the series. The output of the system provides a source of signal of higher voltage, and at a greater impedance level than is available from a single tube of the type selected, while the frequency characteristics approximate those of a well designed amplifying stage using a single tube of the type selected.

What is claimed is:

1. An amplifying system comprising a plurality ofv vacuum tubes, each vacuum tube having a,

In certain types of tubes, it is cathode, control grid and anode, means connecting said tubes in series so that the anode of each tube in the series is connected to the cathode of the following tube in the series, an input circuit connected to the control grid of the first tube in said series, a load resistor system connected to the anode of the last tube in the series, a source of energizing potential connected between the cathode of the first tube in the series and the load resistor system, bias means for said grids, connections from said load resistor system to the control grids of the tubes above the first tube in the series, said connections being so chosen that the amplitude of signals impressed upon all control grids is substantially the same, said system also having a condenser connected between a point on the load resistor system on the one hand and between the anode of one tube and the cathode of the next tube in the series on the other hand, each tube beyond the first in the series having such a condenser and wherein the point of connection on the load resistor system is so selected as to divide said load resistor system into separate load resistors for each tube.

2. In an amplifying system, a plurality of vacuum tubes, each tube having cathode, control grid and anode, means for connecting said tubes in series with the anode of each tube in the series connected to the cathode of the next tube in the series, a bias resistance between the anode of one tube and the cathode of the next tube in the series, an input circuit to the control grid of said first tube in the series, a load having one terminal connected to the anode of the last tube in the series, a. source of potential connected between the other terminal of the load and the cathode of the first tube in the series, connections from said load to the control grids of the tubes above the first tube in the series, each such connection including a blocking condenser and being made to a point on the load where the amplitude of voltage variation is such as to provide signals on the control grids substantially equal in amplitude to the signals on the grid of the first tube, and an output circuit across said load.

3. The system of claim 2 wherein a condenser is connected between each cathode and a point on the load whose potential variation is substantially the same as that of the cathode.

4. An amplifying system comprising a plurality of vacuum tubes, each vacuum tube having a cathode, control grid and anode, means connecting said tubes in series so that the anode of each tube in the series is connected to the cathode of the following in the series, an input circuit connected to the control grid of the first tube in said series, a load resistor connected to the anode of the last tube in the series, a condenser connected between a point on the load resistor on the one hand and between the anode of one tube and the cathode of the next tube in the series on the other hand, each tube beyond the first in the series having such a condenser, the point of connection on the load resistor being so chosen as to divide said load resistor into separate load resistances for each tube, a source of energizing potential connected between the cathode of the first tube in the series and the load resistor, bias means for said grids, and connections from said load resistor to the control grids of the tubes above the first tube in the series, said connections being so chosen as to feed signals of desired amplitude and phase to said control grids.

5. In an amplifying system, a plurality of vacuum tubes, each tube having cathode, control grid and anode, means for connecting said tubes in series with the anode of each tube in the series connected to the cathode of the next tube in the series, an input circuit to the control grid of said first tube in the series, a load connected to the anode of each tube in the cathode-anode circuit thereof, means for impressing input signals on the control grids of tubes above the first in the series from points on the load, the input for any tube bein derived from the load of the preceding tube in the series with the points on said load being so chosen as to provide input signals of equal amplitudes for all the tubes, means for biasing said grids relative to said cathodes, means for impressing suitable energizing potentials on the anodes of said tubes and an output circuit connected between the cathode of said first tube in the series and a point on the load of the last tube in the series, said vacuum tubes being similar and operated similarly.

ROBERT C. MOORE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,052,986 Nyman Sept. 1, 1936 2,093,078 Heising Sept. 14, 1937 2,138,891 Soller Dec. 6, 1938 FOREIGN PATENTS Number Country Date 349,483 Great Britain May 26, 1931 

